Blade assembly for a grass cutting mobile robot

ABSTRACT

A grass cutting mobile robot includes a body and a blade assembly connected to the body and rotatable about a drive axis. The blade assembly includes blades, a housing to hold the blades, and a spring that connects the blade to the housing. The housing includes a slot in which to mount a blade so that a portion of the blade is movable through the slot towards another blade in response to an impact. The slot slopes upwards in the housing towards the body, thereby enabling the blade to move upwards relative to a ground surface toward the body in response to the impact. The spring is for constraining movement of the blade relative to the housing.

TECHNICAL FIELD

This disclosure relates generally to a blade assembly for a grasscutting mobile robot.

BACKGROUND

A mobile lawn mowing robot can navigate about an environment to mow aconfined area. The mobile lawn mowing robot includes blades that arerotatable. The mobile lawn mowing robot can rotate the blades as themobile lawn mowing robot travels along a ground surface through theenvironment. As the blades rotate and contact mowable vegetation, suchas grass, on the ground surface, the blades cut the vegetation.

SUMMARY

In one aspect, the present document features a grass cutting mobilerobot including a body and a blade assembly connected to the body androtatable about a drive axis. The blade assembly includes two or morespring mounted blades, two or more springs, and a housing to hold theblades. Each blade is rotatably mounted on a mounting axis and includesa cutting portion extending inwardly toward the drive axis from a bladetip. Each spring is configured to constrain movement of an associatedone of the two or more blades. The housing includes two or more slots inwhich to mount the two or more blades. The slots are angled so that, inresponse to an impact, a portion of each blade is configured to movewithin a corresponding slot towards the drive axis by rotating about themounting axis of the blade to cause the cutting portion of the blade tomove upward relative to a ground surface toward the body and to reduce atip radius defined by the blade tip and the drive axis as the blade tiprotates about the drive axis.

In another aspect, this document features a grass cutting mobile robotincluding a body and a blade assembly connected to the body androtatable about a drive axis. The blade assembly includes blades, ahousing to hold the blades, and a spring that connects the blade to thehousing. The housing includes a slot in which to mount a blade so that aportion of the blade is movable through the slot towards another bladein response to an impact. The slot slopes upwards in the housing towardsthe body, thereby enabling the blade to move upwards relative to aground surface toward the body in response to the impact. The spring isfor constraining movement of the blade relative to the housing.

In a further aspect, this document features a blade assembly for a grasscutting mobile robot including blades, a housing to hold the blades, anda spring that connects the blade to the housing. The housing isconfigured for coupling to an actuator of the grass cutting mobile robotso that the housing is rotatable about a drive axis. The housingincludes a slot in which to mount a blade so that the blade is movablethrough the slot towards another blade in response to an impact. Theslot slopes upwards in the housing toward a body of the grass cuttingmobile robot, thereby enabling the blade to move upwards in response tothe impact. The spring is for constraining movement of the bladerelative to the housing.

The devices, blade assemblies, and robotic systems described herein mayinclude, but are not limited to, the implementations described below andelsewhere herein. In some examples, the cutting portion can include alength between 10% and 30% of a distance between the mounting axis andthe blade tip.

In some examples, each slot can extend, from proximate the mountingaxis, away from the mounting axis and upward at an incline relative to ahorizontal ground surface. An angle of the incline relative to thehorizontal ground surface can be between 5 and 10 degrees.

In some examples, the mounting axis and the drive axis are non-parallel.

In some examples, the spring can be a torsion spring having a first endcoupled to the housing and a second end coupled to the blade. Thetorsion spring can have a twist axis. The blade can be configured torotate relative to the housing about a mounting axis coincident with thetwist axis and non-parallel to the drive axis.

In some examples, the spring can bias the blade away from the otherblade.

In some examples, absent the impact, a tip radius of the blade can bepositioned to rotate throughout a first radius. In response to theimpact and movement of the blade, the tip radius can be reduced toward asecond radius. The second radius can be less than the first radius.

In some examples, the blade can include a first edge and a second edgeconnected by a surface. The blade can be tilted upward relative to theground surface at a tilt angle such that the second edge is higher thanthe first edge relative to the ground surface. The tilt angle can bebetween 5 degrees and 10 degrees.

In some examples, the blade can include a first portion and a secondportion. The first portion can extend through the slot in the housing.The second portion can extend downward away from the first portion. Theblade can include a third portion extending along a radial axis of theblade assembly.

In some examples, a surface of the blade facing the body can include anembossment extending longitudinally along the surface.

In some examples, the grass cutting mobile robot can include a bumpermounted to the housing. The bumper can have a first height relative tothe ground surface. The blade can have a second height relative to theground surface. The first height can be less than the second height.

In some examples, when the blade assembly is configured to rotaterelative to the body in a first direction, the blade can be configuredto rotate relative to the body in a second direction opposite the firstdirection in response to the impact. The grass cutting mobile robot canfurther include an actuator mounted in the body to rotate the bladeassembly and one or more processors to execute instructions to performoperations. The operations can include detecting an increase in anelectrical current delivered to the actuator and reducing the electricalcurrent delivered to the actuator in response to detecting the increase.The increase can be responsive to the impact.

In some examples, the housing can be configured to receive a shaft thatconnects the housing to the actuator. The shaft can define a groovetherein. The blade assembly can further include a retention clip withinthe housing. The retention clip can include arms. The arms can beslidable within the housing to cause the arms to bend and thereby movetowards, or away from the groove. The arms can be positionable withinthe groove to lock the housing to the actuator.

In some examples, the retention clip can include a tab connecting thearms. The housing can confine the tab and the arms along a plane. Thearms can be configured to slide along the housing and deform outwardlyrelative to the drive axis when a pull force on the tab is directedalong the plane and outward from the drive axis. The arms can beresilient such that the arms can be configured to slide along thehousing and deform inwardly relative to the drive axis when the pullforce on the tab is released.

In some examples, the arms can each include a first end, a second end,and a retaining portion connecting the first end and the second end. Thefirst end can be configured to contact and slide along a post of thehousing. The second end can be configured to contact a support boss ofthe housing. The retaining portion can be positionable within the grooveto lock the actuator to the housing.

In some examples, the housing can include a splined cavity. The splinedcavity can configured to mate with a corresponding splined portion ofthe shaft of the actuator.

In some examples, the blade can be configured to rotate in both a firstdirection and a second direction within the corresponding slot towardsthe drive axis. The spring can be an extension spring configured toextend when the blade rotates in the first direction and configured tocompress when the blade rotates in the second direction.

Advantages of the devices, blade assemblies, and robotic systemsdescribed herein may include, but are not limited to, the following. Theblade assembly can reduce the risk of damaging the blades of the bladeassembly and the actuator of the grass cutting mobile robot. Forexample, because the blades can move relative to the housing of theblade assembly, the blades can contact an object on the ground surfaceand move relative to the housing to maneuver about the object andthereby move away from the object. The blades can move both laterallyand vertically to discontinue the contact with the object and hence beable to move about objects of varying geometries.

The movement of the blades through the slots defined by the housing candecrease an impulse force on the blade assembly when the blades contactthe object. For example, the movement of the blades through the slotscan increase a duration of time over which the force from the contactwith the object occurs, providing a greater amount of time for acontroller of the grass cutting mobile robot to respond to the impact.Similarly, the springs coupled to the blades can produce a biasing forceagainst the force of the impact with objects so that the movement of theblades through the slot occurs over a greater duration of time, furtherincreasing the amount of time for the controller to respond. Decreasedimpulse also can reduce the risk of fatigue and other mechanical failureof the blades.

The retention mechanism of the blade assembly provides a releasemechanism that can enable easy attachment and detachment of the bladeassembly to the actuator of the grass cutting mobile robot. Theretention mechanism can be confined within the housing such that theretention mechanism is easily accessible by a user yet also confinedsuch that the risk of inadvertent disconnection of the blade assemblyfrom the actuator can be decreased.

Any two or more of the features described in this specification,including in this summary section, can be combined to formimplementations not specifically described herein.

The blade assemblies, robotic systems, devices, and techniques describedherein, or portions thereof, can be controlled by a computer programproduct that includes instructions that are stored on one or morenon-transitory machine-readable storage media, and that are executableon one or more processing devices to control (e.g., to coordinate) theoperations described herein. The robots described herein, or portionsthereof, can be implemented as all or part of an apparatus or electronicsystem that can include one or more processing devices and memory tostore executable instructions to implement various operations.

The details of one or more implementations are set forth in theaccompanying drawings and the description herein. Other features andadvantages will be apparent from the description and drawings, and fromthe claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a side view of an example of a grass cutting mobile robot witha blade assembly moving across a ground surface.

FIG. 2 is a bottom view of the grass cutting mobile robot of FIG. 1.

FIG. 3 is an exploded top perspective view of an actuator and a bladeassembly.

FIG. 4 is a side view of the blade assembly isolated from the grasscutting mobile robot of FIG. 1.

FIG. 5 is a bottom perspective view of the blade assembly of FIG. 4.

FIG. 6 is an exploded bottom perspective view of the blade assembly ofFIG. 4.

FIG. 7A is a cross-sectional view of the blade assembly taken along thesection line 7A-7A in FIG. 5 with a blade of the blade assembly in anextended position.

FIG. 7B is the cross-sectional view of the blade assembly shown in FIG.7A with a blade of the blade assembly in a retracted position.

FIG. 8 is a bottom perspective view of a blade, shown transparent,coupled to a spring in the blade assembly.

FIG. 9A is a top perspective view of an example of a blade.

FIG. 9B is a side view of the blade of FIG. 9A.

FIG. 9C is a top view of the blade of FIG. 9A.

FIG. 9D is a cross sectional view of the blade taken along the sectionline 9D-9D shown in FIG. 9C.

FIG. 10 is a schematic top view of the blade assembly showing a blade ofthe blade assembly in the extended position and the retracted position.

FIG. 11 is a top perspective view of the blade assembly showing theblade in the extended position and the retracted position.

FIG. 12 is a side cross-sectional view of the blade assembly isolatedfrom the grass cutting mobile robot taken along the section line 12-12in FIG. 11 with the blade in the retracted position.

FIG. 13 is a side view of the blade assembly isolated from the grasscutting mobile robot with the blade in the retracted position.

FIGS. 14A to 14D are side perspective views of an example of a blade ofa blade assembly contacting an object.

FIGS. 15A to 15D are schematic side views of an example of a blade of ablade assembly contacting an object.

FIG. 16 is a top perspective view of a retention clip.

FIG. 17A is a top view of the blade assembly showing the retention clipof FIG. 16 in a retaining position.

FIG. 17B is a top view of the blade assembly showing the retention clipof FIG. 16 in a release position.

FIG. 18A is a top schematic view of an example of a blade assembly withblades having two cutting edges.

FIG. 18B is a side schematic view of the blade assembly of FIG. 18A.

Like reference numerals in different figures indicate like elements.

DETAILED DESCRIPTION

Described herein are example mobile robots configured to traversemowable areas to cut grass and other vegetation (hereafter referred to,collectively, as grass) using a blade assembly. The blade assembly ismountable on the mobile robot and, in an example implementation,includes two or more spring mounted blades. A quick-release retentionmechanism is configured to enable a user to apply a pull force on theretention mechanism to mount and dismount the blade assembly to themobile robot.

When the blade assembly is mounted on the mobile robot, the mobile robotrotates the blade assembly to cut the grass. In some implementations,the blades are mounted in the blade assembly such that the bladescollapse in response to impact with non-mowable objects in the mowablearea. The collapse of the blades enables the blades to move aroundobjects as the mobile robot continues to traverse the area and the bladeassembly continues to rotate.

FIG. 1 depicts a side view of a grass cutting mobile robot 100 (hereinalso referred to as robot) travelling across a ground surface 50 in aforward direction 102. The ground surface 50 includes unmowed grass 55and mowed grass 60 that is cut by a blade assembly 106. The bladeassembly 106, as shown in FIG. 2, includes a housing 120 in which blades108 are mounted. The grass cutting mobile robot 100 also includes anactuator 112 to which the blade assembly 106 is mounted. The actuator112, when the blade assembly 106 is mounted to the actuator, isconfigured to rotate the blade assembly 106. A small non-mowable object65 (e.g., a small rock) and a large non-mowable object 70 (e.g., a largerock) are in the forward direction 102 of the grass cutting mobile robot100. In some implementations, the robot 100 includes two or more bladeassemblies 106.

Contact between the blade assembly 106 and such objects is undesirable,in some examples, because the contact may damage the blades 108, theblade assembly 106, or an actuator 112 used to drive the rotation of theblades 108. Furthermore, contact between a housing 120 of the bladeassembly 106 may produce a lateral force on the actuator 112, which maydamage a shaft of actuator 112.

The robot 100 includes multiple mechanisms to avoid damage to the robot100 that may be caused by contact with the non-mowable objects 65, 70.As shown in FIG. 2, depicting a bottom view of the robot 100, the robot100 includes a bumper 104 mounted on a forward portion of the body 103of the robot 100. The bumper 104 and the blades 108 are configured tocontact objects on the ground surface 50 as the robot 100 moves aboutthe ground surface 50. Contact between the bumper 104 and objects tallenough to depress the bumper 104 is used to redirect the robot 100 awayfrom larger non-mowable objects (e.g., non-mowable objects having aminimum height of the height of the bottom of the bumper 104). Thebumper 104 protects the blade assembly 106 from contacting these largerobjects.

The blade assembly 106 is mounted on a bottom portion of the body 103and includes a blade retraction mechanism that enables the blades 108 toretract and rise in response to contact with the non-mowable objects.Objects that the bumper 104 does not contact can, in some cases, comeinto contact with the blades 108. In some implementations, as describedherein, blades 108 are mounted in the blade assembly 106 such that, asthe blades 108 contact the object, the blades 108 rotate relative to thehousing 120 of the blade assembly. Referring briefly to FIG. 10, theblades 108 a, 108 b, 108 c (collectively referred to as blades 108) aremounted so that each blade 108 a, 108 b, 108 c, upon contact with anobject, independently rotates relative to the housing 120 of the bladeassembly 106 from an extended position to a retracted position. FIG. 10depicts the blade 108 a in an extended position 174 shown in solidlines. The blade 108 a is rotatable relative to the housing 120 to aretracted position 176 shown in dotted lines. Each of the blades 108 a,108 b, 108 c has a corresponding extended position and retractedposition and is able to rotate between these positions independent ofthe other blades.

This rotation reduces the force of impact resulting from contact betweenthe blades 108 and the object, thereby potentially reducing damage tothe blades 108, the blade assembly 106, and the actuator 112. Asdescribed in greater detail herein, the movement of the blade 108 a tothe retracted position 176 further enables the blade 108 a to move to aposition such that the blade 108 a avoids further contact with theobject. The blade 108 a, for example, rises vertically relative to theground surface 50 to allow the object to pass beneath the blade 108 a.The blade 108 a additionally or alternatively collapses inwardly toallow the object to pass outside of the outermost point (e.g., a bladetip 139 d shown in FIG. 10) of the blade 108 a.

The blade 108 a is also spring-mounted such that the movement of theblade 108 a against the force of a spring delays the transfer of a largeimpulse force directly to the housing 120, thereby decreasing theimpulse force on the actuator 112. The spring furthermore absorbs energyimparted onto the blade 108 a when the blade 108 a when the blade 108 astrikes an object, and biases the blade 108 a back to a cutting positionso that the blade 108 a returns to the cutting position after the blade108 a has cleared the struck object.

As shown in FIGS. 1 and 2, drive wheels 110, in conjunction with casterwheels 111 on a forward portion of the robot 100, support the body 103above the ground surface 50. The robot 100 further includes the actuator112 on which the blade assembly 106 is mounted. Shown in the explodedview of the actuator 112 and the blade assembly 106 depicted in FIG. 3,the actuator 112 includes a shaft 113 on which the blade assembly 106 ismounted.

The robot 100 includes a controller 114 to control operations of systemsof the robot 100. The controller 114, for example, controls one or moremotors that rotate drive wheels 110 of the robot 100 to move the robot100 across the ground surface 50. The controller 114 also controls anamount of power delivered to the actuator 112 to rotate the actuator 112and, when the blade assembly 106 is mounted to the actuator 112, theblade assembly 106.

As the robot 100 moves around the ground surface 50 in the forwardmovement direction 102 as shown in FIG. 1, the bumper 104 of the robot100 is positioned on the body 103 of the robot 100 to contact objectsalong the ground surface 50. When the bumper 104 contacts the object, inresponse to the force of the impact with the object, the bumper 104moves in a rearward direction relative to the body 103 of the robot 100.In some implementations, the bumper 104 also moves in an upwarddirection relative to the body 103 of the robot 100.

The controller 114 alternatively or additionally controls an amount ofpower delivered to the motors rotating the drive wheels 110 and/or theactuator 112 in response to impact between the bumper 104 and theobjects in the environment. In some implementations, the bumper 104includes a contact sensor, force sensor, or other appropriate sensorthat generates signals in response to impact or contact with objects onthe ground surface 50. The controller 114 controls the navigation of therobot 100 depending on the signals generated by the sensor. For example,in response to detecting the contact with an object, the controller 114decreases power delivered to the drive wheels 110 to reduce their speedsor differentially drive the drive wheels 110 to turn the robot 100 awayfrom the object.

In some examples, the bumper 104 contacts objects that have a heightgreater than a bumper height 116 as measured from the ground surface 50to a bottom surface of the bumper 104. As the robot 100 moves in theforward direction 102, the bumper 104 contacts the large object 70 butdoes not contact the small object 65 because the large object 70 has agreater height than the bumper height 116 and the small object 65 has asmaller height than the bumper height 116. The bumper height 116 is, forexample, between 3 and 7 centimeters (e.g., 4 to 6 centimeters,approximately 5 centimeters). A height of the large object 70 can begreater than the bumper height 116 (e.g., greater than 3 to 7centimeters), and a height of the small object 65 can be less than thebumper height 116 (e.g., less than 3 to 7 centimeters).

An object, if sufficiently small, may enter into the underside area 118beneath the body 103. In some examples, when the bumper 104 does notcontact the small object 65, as the robot 100 moves in the forwarddirection 102, the small object 65 moves into an underside area 118beneath the body 103. In some cases, the robot 100 contacts an objecthaving a greater height than the bumper height 116, and the objectcontacts the bumper 104 and cause the bumper 104 to move in the upwarddirection relative to the body 103 such that the object moves beyond thebumper 104 into the underside area 118 beneath the body 103.

The blade assembly 106 is mounted on the robot 100 such that the blades108 are positioned at a blade height 132 above the ground surface 50.The blade height 132 determines the height of the mowed grass 60. Inthis regard, the blade height 132 is selected such that the height ofgrass after the robot 100 mows the grass (e.g., the unmowed grass 55) isat a desired height. In some examples, the blade height 132 is less thanthe bumper height 116, while in other examples, the blade height 132 isgreater than the bumper height 116.

FIG. 4 shows the blade assembly 106 isolated from the robot 100 of FIG.1, and FIG. 5 shows a bottom perspective view of the blade assembly 106.The blade assembly 106 includes the housing 120 to hold the blades 108.While three blades 108 are shown, in some examples, one, two, four, ormore blades are mounted in the blade assembly 106. The housing 120 is,for example, a cylindrical housing. The housing 120 includes slots 122formed between an upper housing 124 and a lower housing 126 that areconnected to form the housing 120 (see FIG. 6) and to define the slots122 in which the blades 108 are mounted.

The blades 108 also move through the slots 122 in response to impactwith objects on the ground surface 50. In some examples, as shown inFig.4, the slots 122 include widened portions 122 a to accommodate theblades 108 in their fully retracted positions. The slots 122 are alsoinclined along the cylindrical housing so that the slots 122 rise asthey extend through the housing, thereby enabling the blades 108 to riseas they move through the housing. As shown in FIGS. 7A and 7B, eachblade 108 is mounted on a pivot shaft 134 in the housing 120 such thateach blade 108 is rotatable relative to the housing 120 about a mountingaxis 135 defined by the pivot shaft 134. For example, as the blade 108 arotates about the mounting axis 135 relative to the housing 120, e.g.,in response to an impact force on the blade, the blade 108 a movesthrough the corresponding slot 122.

The housing 120 further includes a confinement plate 128 that cooperateswith the upper housing 124 to align the actuator 112 of the robot 100 tothe blade assembly 106. The confinement plate 128 and the upper housing124 rotationally couple the actuator 112 to the blade assembly 106. Theupper housing 124 interacts with a retention clip 129, described ingreater detail herein, to lock the actuator 112 to the housing 120. Whenthe actuator 112 is locked to the housing 120, relative translationbetween the actuator 112 and the housing 120 is inhibited, therebyenabling the blade assembly 106 and the shaft 113 of the actuator 112 tojointly rotate about a drive axis 130 relative to the body 103.

The mounting axis 135 of the blades 108 and the drive axis 130 aresubstantially non-parallel. The drive axis 130 is, for example,perpendicular to a horizontal ground surface 50, while the mounting axis135 is angled relative to the drive axis 130. The angle between themounting axis 135 and the drive axis 130 is, for example, is between,for example, 5 and 10 degrees (e.g., between 6 and 9 degrees, 7 and 8degrees, approximately 7.5 degrees); however, as described herein,angles other than these can be used.

Absent impact or contact between objects on the ground surface 50 andthe blades 108, the actuator 112 causes blade tips 173 a, 173 b, 173 c(the blade tip 139 d of the blade 108 a corresponding to the blade tip173 a, and the blade tips 173 a, 173 b, 173 c being collectivelyreferred to as blade tips 173) to rotate within a plane parallel to andabove the ground surface 50. Upon impact or contact between the blades108 and the objects, the blades 108 rotate relative to the housing 120to cause the blade tips 173 to rotate within a plane angled to theground surface 50. The blade tips 173 therefore change in heightrelative to the ground surface 50 as they rotate relative to the housing120 of the blade assembly 106. The angle between the plane of rotationof the blade tips 173 relative to the housing 120 and the plane ofrotation of the blade tips 173 due to the rotation of the actuator 112is between, for example, 5 and 10 degrees (e.g., between 6 and 9degrees, 7 and 8 degrees, approximately 7.5 degrees); however, asdescribed herein, angles other than these can be used. The blades 108rotate such that they move inwardly toward the housing 120 and rise awayfrom the ground surface 50. The blade tips 173, as the blade tips 173rise away from the ground surface 50, are configured to increase up to aheight between, for example, 5 and 15 millimeters (e.g., between 5 and10 millimeters, 10 and 15 millimeters, 6 and 14 millimeters, 7 and 13millimeters, approximately 8 millimeters).

As shown in FIGS. 7A and 7B, example blade 108 a is mounted on thehousing 120 at a tilt angle 137 relative to the ground surface 50. Thetilt angle 137 of the blade 108 a enables the blade 108 a to move in anupward trajectory as the blade 108 a rotates relative the housing 120within the slot 122. In this example, the tilt angle corresponds to theangle formed between the mounting axis 135 and the drive axis 130. Thetilt angle 137 is between, for example, 5 and 10 degrees (e.g., between6 and 9 degrees, 7 and 8 degrees, approximately 7.5 degrees); however,angles other than these may be used as the tilt angle. For example, agreater tilt angle increases the height that the blade 108 a rises asthe blade 108 a move within the corresponding slots 12. In otherexamples, a lower tilt angle in combination with a longer slot increasesthe distance travelled by the blades through the slot 122 withoutdecreasing the maximum height increase of the blade 108 a as the blade108 a moves to the retracted position.

Because the housing 120 is cylindrical, the slot 122 is inclined toenable the blade 108 a at the tilt angle 137 to rotate in the upwardtrajectory relative to the housing 120 through the housing 120. Each ofthe slots 122, for example, is angled to accommodate the tilt angle 137of the blade 108 a. Each slot 122 extends from proximate the mountingaxis 135 away from the mounting axis 135 upward in the housing 120. Eachslot 122 inclines upward toward the body 103 away from the groundsurface 50 to form an angle with the horizontal ground surface 50. Theangle between the slot and the ground surface 50 corresponds to the tiltangle 137 of the blade 108 a. The blade 108 a accordingly are rotatablethrough the slot 122 without contacting a wall surface of the housing120 defining the slot 122.

The blade assembly 106 further includes springs 131 mounted in thehousing 120. For each spring 131, one end of the spring 131 isrotationally constrained to the housing 120 and the other end of thespring 131 is coupled to a corresponding blade 108. Because the spring131 is coupled to both the housing 120 and the blade 108, the spring 131is configured to constrain movement of the corresponding blade 108relative to the housing 120. The spring 131 includes a first end 131 acoupled to the housing 120 and a second end 131 b, as shown in FIG. 8,coupled to the blade 108 a. The second end 131 b of the spring 131 ismounted into, for example, an opening 151 on the blade 108 a.

Because the first end 131 a is coupled to the housing 120 (e.g., thefirst end 131 a is rotationally constrained to the housing 120) and thesecond end is coupled to the blade 108 a, relative motion between thehousing 120 and the blade 108 a causes the spring 131 to twist about thetwist axis 136. The blades 108 are spring-mounted within the housing 120such that the blades 108 are biased into the position as shown in FIGS.3 to 5, 7A, and 11. The spring 131 is positioned such that a twist axis136 of the spring 131 coincides with the mounting axis 135. Both thetwist axis 136 and the mounting axis 135 are angled relative to thedrive axis 130. Each blade 108 has a spring 131 and therefore rotatesindependently from the other blades.

Rotation of the blade 108 a relative to the housing 120 away from itsinitial position through the slot 122 causes the spring 131 to storeenergy. The spring 131 exerts a force opposite of the rotation of theblade 108 a away from its initial position. Thus, as the blade 108 arotates relative the housing 120 toward another blade, the force fromthe spring 131 biases the blade 108 a away from the other blade. Thespring 131 is, for example, a 1 to 5 lb-in (e.g., 1 to 2.5 lb-in, 2.5 to5 lb-in, approximately 2.5 lb-in, 0.11 to 0.57 N-m, 0.11 to 0.28 N-m,0.28 to 0.57 N-m, approximately 0.28 N-m) torsional spring; however,torsional springs having other performance characteristics may be used.For example, a torsional spring with greater strength increases theamount of energy absorbed by the spring when the blade 108 a is rotatedto the fully retracted position and increases the amount of forcerequired to cause the blade 108 a to move to the fully retractedposition. A torsional spring with smaller strength decreases the amountof energy absorbed by the spring when the blade 108 a is rotated to thefully retracted position and decreases the amount of force required tocause the blade 108 a to move to the fully retracted position.

FIGS. 7A and 7B depict a single blade 108 a moving through the housing120. The other blades 108 b, 108 c are not shown to simplify thedepiction of the movement of the blade 108 a. As described herein ingreater detail, the blade 108 a rotates through the slot 122 from anextended position (FIG. 7A) to a retracted position (FIG. 7B). Theextended position corresponds to an initial position of the blade 108 awithin the slot 122. The retracted position corresponds to a finalposition of the blade 108 a within the slot 122 in which the blade 108 acannot rotate farther from its initial position due to contact with thewalls of the housing 120. In the retracted position, a portion of theblade 108 a is within the widened portion 122 a of the slot 122.

FIGS. 9A to 9D illustrate an example of the blade 108 a. The blade 108 adefines a first edge 138 and a second edge 140 that correspond to aleading edge and a trailing edge, respectively, of the blade 108 a asthe blade assembly 106 rotates about the drive axis 130 in a forwardrotating direction 142 (shown in FIG. 5). The first edge 138 and thesecond edge 140 are connected by a top surface 144 and a bottom surface146 of the blade 108 a.

Because the blade 108 a is mounted at the tilt angle 137 relative to theground surface 50, as shown in FIG. 4, the second edge 140 is higherthan the first edge 138 relative to the ground surface 50. The bladeheight 132 corresponds to the height of the first edge 138 above theground surface 50. In this regard, when the blade assembly 106 isrotating, the first edge 138 cuts the unmowed grass 55 to a heightapproximately equal to the blade height 132.

Because the blade 108 a is mounted with the tilt angle 137, the bottomsurface 146 from near the first edge 138 to the second edge 140 ispositioned at a height greater than the blade height 132. Therefore,after the first edge 138 cuts the grass to the blade height 132, thebottom surface 146 clears the mowed grass 60 without dragging along themowed grass 60. Reduced dragging decreases friction forces that need tobe overcome by the actuator 112 as the actuator 112 rotates the bladeassembly 106 to cut the grass. The tilt angle 137 thereby can enablegreater cutting efficiency by inhibiting friction forces that are causedby the dragging of the bottom surface 146.

Each of the blades 108 includes a first portion 139 a, a second portion139 b extending from the first portion 139 a, and a third portion 139 cextending from the second portion 139 b to a blade tip 139 d. The firstportion 139 a is the portion of the blade 108 a mounted within thehousing 120. The first portion 139 a includes the opening 151 to mountthe spring 131 to the blade 108 a as well as an opening 157 to rotatablymount the blade 108 a onto the housing 120 (e.g., to mount the blade 108a onto the pivot shaft 134 of the housing 120). The first portion 139 aextends from within the housing 120, where it is mounted, out of thehousing 120 through the slot 122.

The second portion 139 b of the blade 108 a extends downward from thefirst portion 139 a of the blade 108 a such that the second portion 139b terminates at the blade height 132. The widened portion 122 a of theslot 122 accommodates the second portion 139 b of the blade 108 a whenthe blade 108 a is in the retracted position (FIG. 7B).

The third portion 139 c of the blade 108 a extending from the secondportion 139 b is positioned to extend generally horizontally above theground surface 50 at the blade height 132. The third portion 139 cincludes a cutting portion 138 a of the first edge 138 that extends fromthe blade tip 139 d inwardly toward the drive axis 130. The thirdportion 139 c and the cutting portion 138 a are, for example, coincidentwith or parallel to a radial axis of the blade assembly 106 extendingfrom the drive axis 130. In some cases, the third portion 139 c forms anangle with the radial axis between 0 and 2.5 degrees (e.g., 0 to 1.5degrees, 0 to 0.5 degrees, less than 1 degree, less than 0.5 degrees).

In some examples, because only the third portion 139 c of the blade 108a is at the blade height 132, when the robot 100 travels across theground surface 50 to mow grass, the third portion 139 c contacts thegrass while the first and second portions 139 a, 139 b do not contactthe grass. The third portion 139 c, as the most distal portion of theblade 108 a relative to the drive axis 130 of the blade assembly 106,has a greater lever arm as measured from the drive axis 130. Given atorque applied by the actuator of the robot 100 on the blade assembly106, the force exerted by the third portion 139 c on the grass isgreater than the force that could be exerted by the second portion 139 band the first portion 139 a.

Furthermore, the third portion 139 c has a relatively small lengthcompared to the overall length of the blade 108 a. In some examples, thethird portion 139 c has a length 150 that is a percent of an overallhorizontal length 152 of the blade 108 a between 10% and 50% (e.g.,between 10% and 30%, between 20% and 40%, between 30% and 50%,approximately 20%, approximately 30%, approximately 40%). In someexamples, the horizontal length 152 of the blade 108 a is between 5 and30 centimeters (e.g., between 5 and 7.5 centimeters, between 7.5 and 10centimeters, between 5 and 10 centimeters, between 10 and 20centimeters, between 20 and 30 centimeters, approximately 7.5centimeters, approximately 15 centimeters, approximately 20 centimeters,approximately 22.5 centimeters, approximately 25 centimeters). Thecombination of the smaller surface area of the third portion 139 c incontact with the grass and the longer lever arm of the third portion 139c enables the third portion 139 c to deliver more concentrated force onthe grass as the blade 108 a cuts the grass. The concentrated force, bycutting cleanly through the grass, achieves improved cut quality of thegrass. Improved cut quality, for example, includes achieving a uniformcut over a swath of grass. In some implementations, improved cut qualitymeans that at least between 75-80% of grass blades in a cut area arewithin a range of between 10% to 15% of desired cutting height (e.g.,blade height 132). In some implementations, improved cut quality meansachieving a uniform cut across a vertically oriented blade so that thecut edge is not jagged and/or so that the length of the cut edge is nomore than 10-15% longer than the width of the blade of grass.

Optionally, the top surface 144 of the blade 108 a includes anembossment 148. The embossment 148 extends longitudinally along theblade 108 a. The embossment 148, for example, extends along the topsurface 144 through 50% to 90% (e.g., 60% to 80%, 65% to 75%,approximately 70%) of the horizontal length 152 of the blade 108 a. Asshown in FIG. 9D, which is a cross-sectional view of the blade 108 aalong the section line 9D-9D shown in FIG. 9C, the embossment 148modifies the cross-sectional shape of the blade 108 a to modify air flowacross the blade 108 a as the blade 108 a rotates. The cross-sectionalshape of the embossment 148 is, for example, configured such that airflow during rotation of the blade assembly 106 induces a lift force onthe blade 108 a that would cause the blade 108 a to partially risethrough the slot 122. Such a cross-sectional shape reduces a risk of theblade 108 a bending in response to the air flow that occurs during therotation of the blade assembly 106.

When the blade 108 a moves through the slot 122, the blade 108 a isconfigured such that rotation of the blade 108 a about the mounting axis135 of the blade 108 a through the slot 122 causes the blade 108 a tomove toward the drive axis 130. As depicted in a top view of the bladeassembly 106 in FIG. 10, during cutting operations, the blades 108 a,108 b, 108 c rotate with the blade assembly 106 in a counterclockwisesense (as viewed from above). Each of the blades 108 a, 108 b, 108 c ismovable relative to the housing 120 between an extended position and aretracted position. FIGS. 10 and 11 depict only the blade 108 a in theextended position 174 and in the retracted position 176, but each of theblades 108 a, 108 b, 108 c is movable between these positions.

In some examples, one of the blades 108 contacts objects (e.g., thesmall object 65) as the objects enter the underside area (e.g., theunderside area 118 depicted in FIG. 1) beneath the body 103, causing theblade 108 a to move from the extended position 174 to or toward theretracted position 176. Absent impact or contact with an object on theground surface 50, the blade 108 a is positioned to rotate throughout anextended tip radius 178 (see FIG. 10) corresponding to the tip radius ofthe blade in the extended position 174 as measured from the drive axis130. In the retracted position 176, the blade 108 a rotates through aretracted tip radius 179 (see FIG. 10). In response to the impact andmovement of the blade 108 a, the blade 108 a is positioned to rotatethroughout a radius corresponding to the tip radius of the blade in apartially retracted position between the retracted position 176 and theextended position 174 (e.g., a radius between the extended tip radius178 and the retracted tip radius 179).

As the blade 108 a moves through the slot 122 from the fully extendedposition 174 and the fully retracted position 176, the blade height 132linearly increases with the distance that the blade 108 a travelsthrough the slot 122 toward the fully retracted position 176. Similarly,in some implementations, the tip radius of the blade 108 a alsodecreases linearly with the amount of rotation of the blade 108 athrough the slot 122 toward the fully retracted position 176.

As the blade 108 a is rotating in its extended position 174, the blade108 a is able to avoid further contact with objects positioned in anouter region 175 defined by an outer radius corresponding to theextended tip radius 178 and an inner radius corresponding to theretracted tip radius 179. When the blade 108 a contacts an objectpositioned in the outer region 175, the blade 108 a retracts from theextended position 174 to a partially retracted position within the outerregion 175. In some examples, the blade 108 a retracts from the extendedposition 174 to the fully retracted position 176.

In some examples, the blade 108 a avoids contact with the object in theouter region 175 by rising over the object. In some examples, if anobject enters an inner region 177 defined by a radius corresponding tothe retracted tip radius 179, the blade 108 a is able to avoid furthercontact with the object by rising over the object. Even though the blade108 a cannot retract beyond the retracted tip radius 179, the blade 108a is able to rise high enough to clear the height of the object.Examples of these mechanisms for the blade 108 a to avoid objects aredescribed in greater detail herein, for example, with respect to FIGS.14A to 14D and FIGS. 15A to 15D.

The controller 114 and the blade assembly 106 are configured such thatthe controller reduces the power to the actuator 112 driving the bladeassembly 106 by the time any object enters the inner region 177. Anobject along the ground surface 50 in the forward direction 102 of therobot 100 that enters the inner region 177 contacts one of the blades108 before entering the inner region 177. When one of the blades 108contact the object, because the object contacting the blade 108 causes aforce opposite the direction of rotation of the blade assembly 106, theblade assembly 106 and the actuator 112 experiences a decrease inrotational speed. The decrease in rotational speed of the actuator 112is detectable by the controller 114, for example, using an encoder orother sensor attached to the actuator 112. The controller 114 includes,for example, a feedback speed control mechanism to maintain the speed ofrotation of the actuator 112. The decrease in the detected rotationalspeed of the actuator 112 consequently causes the controller 114,implementing the feedback speed control mechanism, to increase powerdelivered to the actuator 112 of the robot 100 driving the bladeassembly 106 to control the rotational speed to be within apredetermined range.

In some examples, the decrease in rotational speed occurs quicklybecause, as a blade of a blade assembly contacts an object, the impacton the blade is transferred unabated to the housing and then to anactuator driving the blade assembly. The force transfer between theblade and the actuator occurs quickly due to, for example, an absence ofcomponents to absorb or slow the transfer of the impact from the bladeto the actuator. In other examples, the blades contact an object and areunable to move around or above the object as the blade assembly rotates.The actuator driving the blade assembly may stall due to the contactbetween the blades and the object. In some examples, a controller isable to detect a decreased rotational speed of the blade assembly andaccordingly compensates by increasing the power delivered to theactuator driving the blade assembly. However, as the blades 108 areunable to maneuver about the object, the blades 108 remain in contactwith the object, and the power delivered to the actuator 112 maycontinue to increase.

To enable the blades 108 to avoid contact with the object, as describedherein, in response to impact with the object entering the undersidearea, the blades 108 move within the housing 120 from the extendedposition 174 to the retracted position 176. Because the blades are ableto move through the slot 122 against the force of the spring 131, theforce of the impact is absorbed by the spring 131. The impact thereforeoccurs over a greater distance, e.g., the length of the slot 122, thusdecreasing the impulse force on the blade 108 a. The decreased impulseforce can reduce the risk of damaging the blade 108 a.

The impact also occurs over a greater duration of time because the blade108 a initially travels through the slot 122. The greater duration oftime for the impact allows the controller 114 to have a greater amountof time to detect that the rotational speed of the actuator 112 isdecreasing. Upon detecting that the rotational speed is decreasing, thecontroller 114 responds by, for example, ceasing implementation of thefeedback speed control mechanism and initiating a process to stop powerdelivery to the actuator 112. By decreasing the power deliver to theactuator 112, the controller 114 can mitigate a risk of damage to theactuator 112 due to excess power delivered to the actuator 112. Inparticular, the blades 108 are mounted in the housing such that, by thetime the object has moved through the outer region 175 to the innerregion 177, one of the blades 108 has contacted the object for a greatenough duration of time to enable the controller 114 to detect theresulting decrease in the rotational speed of actuator 112. Thecontroller 114 then reduces power to the actuator 112 so that the blades108 do not continue rotating against the object.

In addition, the movement of the blades 108 through the slots 122 alsoprovides the controller 114 with sufficient time to detect the contactwith the object so that the controller 114 can reduce or cut powerdelivered to the drive wheels 110. As a result, any object that entersthe inner region 177 is detected by the controller 114 before the grasscutting mobile robot 100 moves enough to cause the housing 120 of theblade assembly 106 to contact the object. As described herein, thatcontact may damage the actuator shaft 113. The ability of the controller114 to inhibit this contact protects the actuator 112 from this damage.

In some implementations, the controller 114 detects an increase in powersupplied to the actuator 112 to maintain the rotational speed of theactuator 112 and responds to the increase in the power supplied bystopping power delivery to the actuator 112. The increase in the power,for example, corresponds to a spike in power that indicates that one ormore of the blades 108 has struck an object. In some examples, thecontroller 114 detects a mechanical shock on the blade assembly 106and/or the blades 108 based on signals from an accelerometer coupled tothe blades 108 and/or the housing 120 of the blade assembly 106. Anincrease in the measured acceleration of the actuator indicates to thecontroller 114 that the blades 108 and/or the housing 120 have contactedan object. The increase in the measured acceleration, for example,corresponds to a spike in the measured acceleration indicating thecontact between a component of the blade assembly 106 and an object. Thecontroller 114, in response, reduces the power supplied to the actuatoror stops delivering power to the actuator 112.

As described herein, the bumper 104 and the blade assembly 108 eachprovides a mechanism to avoid damage to the robot 100 that may be causedby contact with non-mowable objects. The controller 114 describedherein, for example, uses a combination of the sensing systemsassociated with the bumper 104 and the blade assembly 108 to avoiddamage to the blades 108, the actuator 112, and other components of therobot 100. In some examples, the robot 100 includes a sensor to detectthat an object has contacted the bumper 104 and caused an upwardmovement of the bumper 104. The object, for example, causes a lift inthe body 103, the drive wheels 110, and/or the caster wheels 111 of therobot 100. One or more sensors attached to the body 103, the drivewheels 110, and/or the caster wheels 111 generates an electrical thatcorresponds to an amount of the lift. The sensor is, for example, anaccelerometer, a velocity sensor, a position sensor, a force sensor, orother appropriate sensor that is responsive to the object contacting thebumper 104 and causing an upward force on the body 103 and the bumper104. The upward force is, for example, a result of an applied upwardforce directly on the bumper 104 causing relative motion between thebumper 104 and the body 103 or an upward force causing upward motion ofthe bumper 104 and the body 103 together.

In examples where the sensor is a force sensor, if the force detectedduring a mowing operation is higher than a threshold force, thecontroller 114 responds by discontinuing delivery of power to the drivewheels 110 and/or the actuator 112. If the force is below the thresholdforce, the controller 114 continues the mowing operation withoutadjusting the amount of power delivered. In some cases, the bumper 104contacts the object, and the sensor does not detect a force above thethreshold force. The robot 100 continues moving in the forward drivedirection, which causes the blades 108 and/or the housing 120 to contactthe object.

The movement to the retracted position 176 also enables the blades 108to avoid further contact with the object as the blade assembly 106continues to rotate. The movement of the blades 108 within the housing120 enables the blades 108 to rotate into the housing 120 (decreasingthe tip radius of the blades 108) and to move upward relative to theground surface 50. The rotation of the blades 108 into the housingcauses lateral movement of the blades 108 relative to the object so thatthe blades 108 avoid further contact with the object. The combination ofthe rotation of the blades 108 into the housing and the upward movementof the blades 108 in response to contact with the object preventsfurther contact between the blades 108 and the object.

The movement to the retracted positions causes the radius of the bladetips 173 to decrease by, for example, 20 to 40 millimeters (e.g., 20 to30 millimeters, 30 to 40 millimeters, approximately 30 millimeters). Insome implementations, the retracted tip radius 179 is 40% to 80% (e.g.,40% to 60%, 60% to 80%, 50% to 70%, approximately 50%, approximately60%, approximately 70%) of the extended tip radius 178. In someexamples, the extended tip radius 178 is between 8 and 12 centimeters(e.g., between 9 and 11 centimeters, approximately 10 centimeters), andthe retracted tip radius 179 is between 4 and 8 centimeters (e.g.,between 5 and 7 centimeters, approximately 6 centimeters). Duringoperation, the blades 108 have a radius between the extended tip radius178 and the retracted tip radius 179 (inclusive) depending on the amountof retraction of the blades 108 (e.g., due to contact or absence ofcontact with an object).

The upward movement of the blades 108 relative to the ground surface 50causes an increase in the blade height 132 so that the blades 108 climbover the object. The blade height 132 increases by, for example, 5 to 15millimeters (e.g., 5 to 10 millimeters, 10 to 15 millimeters, 6 to 14millimeters, 7 to 13 millimeters, approximately 8 millimeters). In someexamples, the blade height 132 of the blades 108 in the extendedposition 174 is between 30 millimeters and 50 millimeters (e.g., between30 and 40 millimeters, between 35 and 45 millimeters, between 40 and 50millimeters, approximately 35 millimeters, approximately 40 millimeters,approximately 45 millimeters). The blade height 132 of the blades 108 inthe retracted position 176 is between 40 and 60 millimeters (e.g.,between 40 and 50 millimeters, between 45 and 55 millimeters, between 50and 60 millimeters, approximately 45 millimeters, approximately 50millimeters, approximately 55 millimeters). The ratio of the bladeheight 132 in the retracted position 176 to the blade height 132 in theextended position 174 is, for example, 1.05 to 1.25 (e.g., 1.05 to 1.15,1.10 to 1.20, 1.15 to 1.25, approximately 1.10, approximately 1.15,approximately 1.20).

With respect to the lateral movement of the blades 108 upon impactdescribed herein, when the blade 108 a travels toward the retractedposition 176 from the extended position 174, as shown in FIG. 11, afirst blade 108 a moves through the slot 122 toward a second blade 108 band away from a third blade 108 c. Initially, absent contact withobjects on the ground surface 50, the blades 108 are equally spaced fromone another. The blade tips 173 of the blades 108, in some examples,form 120 degree angles with one another. In some cases, the blade tips173 are equidistantly spaced along a circumference through which theblade tips 173 are swept when the blade assembly 106 is rotated and theblades 108 are each in an extended position.

In some examples, when one of the blades 108 a, 108 b, 108 c, e.g., theblade 108 a, contacts an object, the blade 108 a moves to a partiallyretracted position between the extended position 174 and the retractedposition 176, thereby causing the blade 108 a to become unequally spacedfrom the blades 108 b, 108 c. The angle between the blade tip 173 a ofthe blade 108 a (in the retracted position 176) from the blade tip 173 bof the blade 108 b (in an extended position) is, for example, 30 to 70degrees (e.g., between 30 and 50 degrees, between 40 and 60 degrees,between 50 and 70 degrees, approximately 40 degrees, approximately 50degrees, approximately 60 degrees). The angle between the blade tip 173a of the blade 108 a (in the retracted position 176) and the blade tip173 c of the blade 108 c (in an extended position) is, for example, 150to 240 degrees (e.g., between 150 and 180 degrees, between 180 and 210degrees, between 210 and 240 degrees, approximately 165 degrees,approximately 195 degrees, approximately 225 degrees). In someimplementations, the angle between the blade tip 173 a of the blade 108a and the blade tip 173 b of the blade 108 b (in the extended position)decreases by 25% to 40% (e.g., 25% to 35%, 30% to 40%, approximately30%, approximately 35%) as the blade 108 a moves from the extendedposition 174 to the retracted position 176.

Furthermore, moving from the extended position 174 to the retractedposition 176, the blade 108 a travels through the slot 122 such that theblade 108 a moves toward the drive axis 130. The third portion 139 cmoves inward toward the drive axis 130 such that a tip radius defined bythe blade tip 139 d and the drive axis 130 reduces as the blade 108 atravels from the extended position 174 to the retracted position 176. Asdescribed herein, the extended tip radius 178 when the blade 108 a is inthe extended position 174 is greater than the retracted tip radius 179when the blade 108 a is in the retracted position 176.

With respect to the upward movement of the blades 108 described herein,the blade 108 a also travels through the slot 122 such that the blade108 a moves upward relative to the ground surface 50 toward the body103. In particular, as the blade 108 a moves toward the drive axis 130,the third portion 139 c moves upward relative to the ground surface 50toward the body 103. Because the blade 108 a is mounted to the housingthrough the opening 157 (as shown in FIG. 9A), the opening 157 of theblade 108 a does not move relative to the housing 120. As shown in FIG.12, the blade 108 a is in the retracted position 176. The third portion139 c of the blade 108 a in the retracted position 176 has a retractedblade height 180 that is greater than an extended blade height 181 ofthe blades 108 b, 108 c, which are both in extended positions.

FIGS. 14A to 14D schematically depict a position of the blade 108 a whenthe blade 108 a contacts an object 80 and consequently travels withinthe housing 120 to rise over the object 80 to avoid being stuck incontact with the object 80. FIGS. 14A to 14D show sequential sideperspective views of the blade 108 a as the blade assembly 106 rotatesin a first direction 182 about the drive axis 130 and the blade 108 acontacts the object 80.

In FIG. 14A, the blade 108 a initially contacts the object 80 as theblade assembly 106 rotates in the first direction 182 about the driveaxis 130. The blade 108 a is initially at a blade height 186 a above theground surface 50 and is initially in the extended position. The initialcontact causes a force on the blade 108 a in a second direction 184 thatopposes the first direction 182. The contact with the object 80 causesthe blade 108 a to overcome the spring force of the spring and therebycauses the object to rotate in the second direction 184 toward theretracted position relative to the housing 120. The rotation in thesecond direction 184 causes the blade 108 a to move upward relative tothe ground surface 50.

In FIG. 14B, the blade assembly 106 continues rotating in the firstdirection 182. However, because of the object 80, the blade 108 arotates in the second direction 184 and travels through the slot 122while remaining in contact with the object 80. The continued contactwith the side of the object 80 causes the blade 108 a to travel throughthe slot 122 toward the retracted position. The blade height 186 bincreases because the blade 108 a is tilted at an angle relative to theground surface 50. The blade 108 a moves upwards relative to the groundsurface 50 toward the body 103 of the robot 100 in response to continuedcontact with the object and rotation of the blade assembly 106.

The rotation of the blade 108 a within the slot 122 away from itsinitial extended position causes the spring (e.g., the spring 131) totwist. The spring, as the amount of twisting increases, biases the blade108 a back toward the initial extended position, but the blade 108 a isunable to return to the extended position while the blade 108 a iscontact with the object 80. As a result, the blade 108 a remains incontact with the object 80 in a partially retracted position between theinitial extended position and the fully retracted position.

In FIG. 14C, the blade assembly 106 continues rotating in the firstdirection 182. Because the blade 108 a travels through the slot 122 andconsequently experiences an increase in its blade height, the blade 108a reaches a blade height 186 c sufficient to climb over the object 80.In particular, the blade 108 a reaches a blade height 186 c that isgreater than an object height. While the blade 108 a travels across thetop of the object 80, if the object 80 has a flat top surface, the blade108 a remains in relatively the same position within the slot 122. Thespring continues to bias the blade 108 a back toward the initialextended position, but the top of the object 80 limits the movement ofthe blade 108 a in the direction toward the initial extended position.

In FIG. 14D, the blade assembly 106 continues rotating in the firstdirection 182. The blade 108 a has travelled across the length of thetop of the object 80 and thereby is able to return toward its initialposition within the slot 122 depicted in FIG. 14A. The spring biases theblade 108 a back toward the initial extended position. Because theobject 80 no longer blocks the blade 108 a, the biasing force of thespring is able to cause rotation of the blade 108 a back toward theinitial extended position. In some examples, another blade 108 bcontacts the object after the blade 108 a loses contact with the object80.

In contrast to FIGS. 14A to 14D where the blade 108 a maneuvers beyondan object 80 by climbing over the object, the blade 108 a depicted inFIGS. 15A to 15D beyond an object 90 by lateral movement of the blade108 a relative to the object. In particular, FIGS. 15A to 15Dschematically depict a position of the blade 108 a when the blade 108 acontacts an object 80 and consequently travels within the housing 120 tomaneuver laterally around the object 80 to avoid being stuck in contactwith the object 80. FIGS. 15A to 15D show sequential top views of theblade 108 a as the blade assembly 106 rotates in a first direction 190about the drive axis 130 and the blade 108 a contacts the object 90.

In FIG. 15A, the blade 108 a initially contacts the object 90 as theblade assembly 106 rotates in the first direction 190 about the driveaxis 130. The blade 108 a is initially in the extended position and at ablade tip radius 194 a. The initial contact causes a force on the blade108 a. The force causes the blade to rotate in a second direction 192about the mounting axis 135 relative to the housing 120. The seconddirection 192 of movement opposes the rotation of the blade 108 a (withthe blade assembly 106) in the first direction 190. The contact with theobject 90 causes the blade 108 a to move toward the retracted position,therefore causing the blade 108 a to move toward another blade andinward relative to the drive axis 130.

In FIG. 15B, the blade 108 a rotates in the second direction 192relative to the housing 120 to cause blade 108 a to begin retracting.The blade 108 a rotates within the slot (not shown) relative to thehousing 120 such that the blade tip radius 194 b decreases from theblade tip radius 194 a shown in FIG. 17A. The blade tip radius 194 b ispositioned inward toward the drive axis 130 relative to the blade tipradius 194 a. The rotation of the blade 108 a within the housing 120away from its initial extended position 196 causes the spring (e.g., thespring 131) to twist. The spring, as the amount of twisting increases,biases the blade 108 a back toward the initial extended position 196,but the blade 108 a is unable to return to the extended position 196while the blade 108 a is contact with the object 90. As a result, theblade 108 a remains in contact with the object 90 in a partiallyretracted position between the extended position 196 and the fullyretracted position.

In FIG. 15C, the blade assembly 106 continues rotating. The blade 108 ahas rotated a sufficient amount within the housing 120 toward theretracted position such that the blade tip radius 194 c is less than adistance between the object 90 and the drive axis 130. As shown in FIG.15D, once the blade 108 a has collapsed enough to clear the object 90,the spring biases the blade 108 a back toward the extended position 196.In this regard, the blade 108 a is rotating with the rotation of theblade assembly 106 in the first direction 190 about the drive axis 130and is additionally rotating relative to the housing 120 about themounting axis 135.

In some implementations, the object 80 has a height that the blade 108 ais unable to clear, or the object 90 is positioned sufficiently close tothe drive axis 130 to prevent the blade 108 a from maneuvering about theobject 90. In particular, the blade 108 a moves from its initialextended position to the its fully retracted position. Even in the fullyretracted position, the blade tip radius is too large for the blade 108a to move laterally around the object to avoid the object, or the bladeheight is too small for the blade 108 a to climb over the object toavoid the object.

In these cases, the blade 108 a moves through the slot 122 from theinitial extended position to the fully retracted position as theactuator 112 rotates the blade assembly 106. During this movementthrough the slot 122, the blade 108 a contacts the object 80, 90, whichimparts a force on the blade assembly 106 that would cause the blade 108a to move in a direction opposite the rotation of the blade assembly106. The force is therefore, for example, in opposition to the torquethat the actuator 112 applies on the blade assembly 106. The forcedecreases the speed of the blade assembly 106, and the controller 114,using the feedback speed controls, increases an electrical currentdelivered to the actuator 112 to maintain the rotational speed of theblade assembly 106. The controller 114 then detects this increase in theelectrical current delivered to the actuator 112. Once the increase isbeyond a predetermined threshold, the controller 114 reduces theelectrical current delivered to the actuator 112 to avoid delivering anamount of electrical current beyond the specified maximum allowedcurrent of the actuator 112. In some examples, the controller 114disables the feedback speed controls so that any decrease in rotationalspeed of the actuator 112 does not cause the controller 114 to deliver agreater amount of power to the actuator 112.

Because the blade 108 a is able to move through the slot 122 against theforce of the spring 131, the impact with the object 80, 90 generates aforce that is absorbed by the spring 131. The impact therefore occursover a greater distance, e.g., the length of the slot 122, thusdecreasing the impulse force on the blade 108 a. Decreasing the impulseforce reduces the risk of damaging the blade 108 a.

In addition, because the spring 131 initially absorbs the force, theblade assembly 106 does not experience a sudden decrease in rotationalspeed due to the impact with the object but rather experiences a gradualdecrease in rotational speed. The gradual decrease provides a greateramount of time for the controller 114 to detect the increased currentdelivered to the actuator 112 to compensate for the gradual decrease inthe rotational speed.

For the actuator 112 to rotate the blade assembly 106 as described inthe examples herein, the blade assembly 106 is mounted onto the actuator112 such that the housing 120 of the blade assembly 106 is rotationallyconstrained to the actuator 112. As shown in FIG. 17A, to rotationallyconstrain the housing 120 of the blade assembly 106 to the actuator 112,the shaft 113 of the actuator 112 mates with a splined cavity 153defined by the housing 120. Referring also to FIG. 3, when the bladeassembly 106 is mounted to the actuator shaft 113, the splined cavity153 interfaces with a splined portion 154 of the actuator shaft 113. Thesplined cavity 153 aligns the actuator shaft 113 with the blade assembly106. The splined cavity 153 receives and mates with the splined portion154 of the shaft 113 to limit relative rotation between the actuatorshaft 113 and the blade assembly 106 when the blade assembly 106 isproperly mounted to the shaft 113.

To inhibit relative translation of the housing 120 and the shaft 113 ofthe actuator 112, the blade assembly 106 includes the retention clip 129(FIG. 16) that forms a (quick release) retention mechanism, as depictedin FIG. 17A. The retention mechanism facilitates attachment of the bladeassembly 106 to the shaft 113 of the actuator 112 of the robot 100 so asto translationally constrain the blade assembly 106 to the actuator 112.In particular, a groove 155 of the splined portion 154, as describedherein, engages with the retention clip 129 to translationally constrainthe blade assembly 106 to the actuator shaft 113.

The retention mechanism includes the retention clip 129, which includesa first arm 156 a and a second arm 156 b (collectively referred to asarms 156) connected by a tab portion 158. The housing 120 includes theconfinement plate (e.g., the confinement plate 128 shown in FIG. 6) thatconfines the retention clip 129 between the confinement plate and thehousing 120. The housing 120 further defines upwardly extending posts162 a, 162 b and a support boss 160. The confinement plate and thehousing 120 confine the retention clip 129 such that the tab portion 158and the arms 156 are confined along a plane.

An inset portion 159 within the housing 120 enables the user to manuallypull the tab portion 158. The confinement plate and the inset portion159 of the housing 120 allow only the tab portion 158 to be accessedexternally by the user. The inset portion 159, in some cases, iscounterbalanced by mass removed from the housing 120 on an oppositelateral portion 161 of the housing 120 such that the mass of bladeassembly 106 is axisymmetrically distributed about the drive axis 130.For example, the housing 120 is hollow in the opposite lateral portion161 such that the absence of material of the inset portion 159 isbalanced by an absence of material in the opposite lateral portion 161.

The arms 156 are slidable within the housing 120 along the plane. Asdescribed herein, the user applies a pull force 172 to enable release ofthe retention clip 129 from the actuator 112. The posts 162 a, 162 bextend into the plane such that the arms 156 abut and slide along theposts 162 a, 162 b. The support boss 160 also extends into the plane inwhich the retention clip 129 is confined. Each of the arms 156 includesa support portion 163 a, 163 b, a retaining portion 164 a, 164 b, a stepportion 167 a, 167 b, a first stop portion 166 a, 166 b, a slidingportion 168 a, 168 b, and a second stop portion 170 a, 170 b. Thesupport portions 163 a, 163 b, connecting the tab portion 158 to thearms 156, extend away from the tab portion 158 toward the drive axis130. The support portions 163 a, 163 b, for example, are substantiallyparallel linear portions that extend along the axis of the pull force172.

The retaining portions 164 a, 164 b of the arms 156 are the portions ofthe arms 156 proximate to the drive axis 130, and hence the portionsmounted within the groove 155 of the actuator shaft 113 when the bladeassembly 106 is mounted to the actuator shaft 113. The retainingportions 164 a, 164 b extend from the support portions 163 a, 163 b. Insome examples, the retaining portions 164 a, 164 b extend along the axisof the pull force 172 and include concave portions 165 a, 165 b thathave radii of curvatures that accommodate the radii of curvature of thegroove 155 of the splined portion 154. The radii of curvature of theconcave portions 165 a, 165 b and the groove 155 of the splined portion154 are, for example, between 2 millimeters and 6 millimeters (e.g.,between 2 and 4 millimeters, between 4 and 6 millimeters, approximately3 millimeters, approximately 4 millimeters, approximately 5millimeters).

The step portions 167 a, 167 b extend from the retaining portions 164 a,164 b away from the drive axis 130, thus forming angles with theretaining portions 164 a, 164 b. The first stop portions 166 a, 166 bextend from the step portions 167 a, 167 b along the axis of pull force172. The sliding portions 168 a, 168 b extend and are angled away fromthe first stop portions 166 a, 166 b. The sliding portions 168 a, 168 bextend toward one another. The second stop portions 170 a, 170 b extendfrom the sliding portions 168 a, 168 b toward the drive axis 130. Insome implementations, the second stop portions 170 a, 170 b aresubstantially linear and parallel and extend along the axis of the pullforce 172 toward the drive axis 130.

The extension of the step portions 167 a, 167 b away from the drive axis130 enables the first stop portions 166 a, 166 b to be positionedfarther from one another while enabling the retaining portions 164 a,164 b to be positioned closer to one another. The step portions 167 a,167 b accordingly are sized and dimensioned to define the distancebetween the first stop portions 166 a, 166 b and the distance betweenthe retaining portions 164 a, 164 b. The retaining portions 164 a, 165 bare positioned such that, in a retaining position (FIG. 17A), theconcave portions 165 a, 165 b engage the groove 155 of the actuatorshaft 113, and in a release position (FIG. 17B), the concave portions165 a, 165 b do not engage the groove 155 of the actuator shaft 113. Thedistance between the first stop portions 166 a, 166 b, in turn definethe lengths of the sliding portions 168 a, 168 b, which at least in partdetermine the amount of separation that occurs due to the movement ofthe retaining clip 129 from the retaining position (FIG. 17A) to therelease position (FIG. 17B).

In some examples, the lengths of the support portions 163 a, 163 b arebetween 4 and 6 millimeters. The lengths of the retaining portions 164a, 164 b are between, for example, 7 and 11 millimeters. The lengths ofthe step portions 167 a, 167 b are between, for example, 0.5 and 3millimeters. The angle formed between the step portion 167 a, 167 b andthe retaining portion 164 a, 164 b is, for example, between 120 and 150degrees. The lengths of the first stop portions 166 a, 166 b are, forexample, between 1 and 4 millimeters. The lengths of the slidingportions 168 a, 168 b are, for example, between 2 and 6 millimeters. Theangle formed between the sliding portion 168 a, 168 b and the first stopportion 166 a, 166 b is, for example, between 130 and 170 degrees. Thelengths of the second stop portions 170 a, 170 b are, for example,between 1 and 4 millimeters.

The alignment mechanism between the actuator shaft 113 and the bladeassembly 106, while described as an interface between the splined cavity153 and the splined portion 154, is a lock-and-key, an offset boss, orother appropriate mechanism to rotationally constrain the actuator shaft113 to the blade assembly 106. The portion 154, for example, includesone or more longitudinally extending posts that mate with cavitiesdefined by the housing 120 of the blade assembly 106. The posts matedwith the cavities inhibit relative rotational movement between thehousing 120 and the shaft 113. In some cases, the shaft 113 includes aradially extending flange rotationally asymmetric about the drive axis130. The radially extending flange inserts into a corresponding cavityin the housing 120 to rotationally couple the blade assembly 106 to theactuator shaft 113.

FIG. 17A shows the retention clip 129 in the retaining position. In theretaining position, the arms 156 contact the support boss 160 at one endand contact the posts 162 a, 162 b at the other end. In particular, thesupport portions 163 a, 163 b abut the support boss 160, and the firststop portion 166 a, 166 b of each arm 156 abuts the corresponding post162 a, 162 b. If the splined portion 154 of the actuator shaft 113 hasbeen inserted into the splined cavity 153, the retaining portion 164 a,164 b of each of the arms 156 interfaces with the splined portion 154 toprevent relative translation (e.g., relative vertical movement) betweenthe blade assembly 106 and the shaft 113. For example, in thisimplementation, the retaining portions 164 a, 164 b are positionedwithin the groove 155 of the splined portion 154 to lock the housing 120and the blade assembly 106 to the actuator 112.

The retention clip 129 is movable between the retaining position (FIG.17A) and the release position (FIG. 17B). When a pull force 172 appliedon the tab portion 158 is directed along the plane to which theretention clip 129 is confined and is directed outward from the driveaxis 130, the arms 156 slide along the housing 120 to increase aseparation distance between the retaining portions 164 a, 164 b. Theuser, for example, applies the pull force 172 on the tab portion 158 bypulling the tab portion 158 away from the drive axis 130, therebycausing the retaining portions 164 a, 164 b to move away from oneanother. The increased distance between the retaining portions 164 a,164 b enables the retaining portions 164 a, 164 b to be removed from thegroove 155 of the actuator shaft 113 such that the actuator 112 istranslatable relative to the housing 120.

During application of the pull force 172, the support portions 163 a,163 b remain in sliding contact with the support boss 160. As a result,the arms 156 deform outwardly relative to the drive axis 130, with thesupport portions 163 a, 163 b remaining substantially undeformed. Thepull force 172 causes the sliding portions 168 a, 168 b to slide alongthe posts 162 a, 162 b, in turn causing the retaining portion 164 a, 164b of the arms 156 to deform outwardly relative to the drive axis 130.With continued application of the pull force 172, the sliding portions168 a, 168 b continue sliding along the posts 162 a, 162 b until thesecond stop portions 170 a, 170 b abut the posts 162 a, 162 b. Duringthis sliding motion, the retaining portions 164 a, 164 b continue todeform outwardly relative to the drive axis 130.

When the second stop portions 170 a, 170 b abut the posts 162 a, 162 b,the arms 156 are in the release position (FIG. 17B). In the releaseposition, the retaining portions 164 a, 164 b are no longer positionedwithin the groove 155 of the splined portion 154. If the blade assembly106 was mounted to the actuator 112, when the retention clip 129 is inthe release position, the blade assembly 106 is no longer locked to theactuator 112. The blade assembly 106 is translatable relative to theactuator 112 such that the blade assembly 106 can be dismounted from theactuator 112.

In a retaining position as depicted in FIG. 17A, the first stop portions166 a, 166 b are positioned between, for example, 7 and 13 millimeters(e.g., between 7 and 9 millimeters, between 9 and 11 millimeters,between 11 and 13 millimeters, approximately 8 millimeters,approximately 10 millimeters, approximately 12 millimeters) away fromone another. The retaining portions 164 a, 164 b are positioned between,for example, 6 and 12 millimeters (e.g., between 6 and 8 millimeters,between 8 and 10 millimeters, between 10 and 12 millimeters,approximately 7 millimeters, approximately 9 millimeters, approximately11 millimeters). Because the sliding portions 168 a, 168 b extend towardone another, in some examples, in the retaining position (FIG. 17A) ofthe retention clip 129, the second stop portions 170 a, 170 b areadjacent one another. The second stop portions 170 a, 170 b are, forexample, between 1 millimeter and 1.5 millimeters away from one another(as measured from a longitudinal axis of the second stop portion 170 ato a longitudinal axis of the second stop portion 170 b). In someexamples, the second stop portions 170 a, 170 b are contacting oneanother when the retention clip 129 is in the retaining position (FIG.17A).

In the release position as depicted in FIG. 17A, the first stop portions166 a, 166 b are positioned between, for example, 12 and 18 millimeters(e.g., between 12 and 14 millimeters, between 14 and 16 millimeters,between 16 and 18 millimeters, approximately 13 millimeters,approximately 15 millimeters, approximately 17 millimeters) away fromone another. The retaining portions 164 a, 164 b are positioned between,for example, 10 and 16 millimeters (e.g., between 10 and 12 millimeters,between 12 and 14 millimeters, between 14 and 16 millimeters,approximately 11 millimeters, approximately 13 millimeters,approximately 15 millimeters) away from one another. The second stopportions 170 a, 170 b are, for example, 5 to 7 millimeters away from oneanother.

When the retention clip 129 is moved from the retaining position (FIG.17A) to the release position (FIG. 17B), in some implementations, thedistance between the first stop portions 166 a, 166 b increases by 50%to 150% (e.g., 50% to 100%, 100% to 150%). In some cases, the distancebetween the retaining portions 164 a, 164 b increases by 40% to 80%(e.g., 40% to 60%, 60% to 80%). In some examples, the distance betweenthe second stop portions 170 a, 170 b increases by 300% to 700% (e.g.,between 300% and 500%, between 500% and 700%).

When the first stop portions 166 a, 166 b contact the support posts 162a, 162 b, the arms 156 extend substantially parallel to the axis of thepull force (e.g., the support portions 163 a, 163 b, the first stopportions 166 a, 166 b, and the second stop portions 170 a, 170 b extendsubstantially parallel to the axis of the pull force 172). Thee supportportions 163 a, 163 b, the first stop portions 166 a, 166 b, and thesecond stop portions 170 a, 170 b, for example, each form an anglebetween 0 and 2.5 degrees with the axis of the pull force 172. When thearms 156 are in a fully deformed position (e.g., when the second stopportions 170 a, 170 b contact the support posts 162 a, 162 b, as shownin FIG. 17B), the arms 156 deform at an angle relative to the axis ofthe pull force 172. The angle when the arms 156 are deformed is, forexample, between 5 and 15 degrees (e.g., between 7 and 13 degrees,between 9 and 11 degrees, approximately 8 degrees).

The arms 156 are formed of a resilient material, such as, for example,aluminum, stainless steel, acetal, or other resilient material. As aresult, when the pull force 172 is released, the retention clip 129returns to the retaining position (FIG. 17A). In some implementations,the arms 156 are further coupled to a spring or other resilient memberthat biases the arms 156 toward the retaining position.

When the retention clip 129 locks into the groove 155, in some examples,the retention clip 129 contacts the shaft 113 and generates an audibleand tactile indication that the retention clip 129 is properly seatedinto the groove 155. The audible indication is, for example, a clickingnoise that indicates to the user that blade assembly 106 is coupled tothe actuator 112. In some cases, the housing 120 includes one or moreprotrusions that contact the arms 156 of the retention clip 129 as theretention clip 129 moves to the retaining position (FIG. 17A). Thecontact between the protrusions and the arms 156 generates an additionalaudible indication that the retention clip 129 is in the retainingposition (FIG. 17A). In further examples, the housing 120 includes oneor more protrusions that contact the arms 156 as they move into therelease position (FIG. 17B), thereby enabling yet another audibleindication that informs a user that the retention clip 129 has beenreleased.

One or more controllers (e.g., the controller 114) may control all orpart of the foregoing operation of the grass cutting mobile robot byexecuting one or more computer programs. A computer program can bewritten in any form of programming language, including compiled orinterpreted languages, and it can be deployed in any form, including asa stand-alone program or as a module, component, subroutine, or otherunit suitable for use in a computing environment.

Operations associated with implementing all or part of the controlprocesses, for example, for the actuator 112, described herein can beperformed by one or more programmable processors executing one or morecomputer programs to perform the functions described herein. Controlover all or part of the control processes described herein can beimplemented using special purpose logic circuitry, e.g., an FPGA (fieldprogrammable gate array) and/or an ASIC (application-specific integratedcircuit).

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor will receive instructions and data from a read-only storagearea or a random access storage area or both. Elements of a computerinclude one or more processors for executing instructions and one ormore storage area devices for storing instructions and data. Generally,a computer will also include, or be operatively coupled to receive datafrom, or transfer data to, or both, one or more machine-readable storagemedia, such as mass PCBs for storing data, e.g., magnetic,magneto-optical disks, or optical disks. Machine-readable storage mediasuitable for embodying computer program instructions and data includeall forms of non-volatile storage area, including by way of example,semiconductor storage area devices, e.g., EPROM, EEPROM, and flashstorage area devices; magnetic disks, e.g., internal hard disks orremovable disks; magneto-optical disks; and CD-ROM and DVD-ROM disks.

While the housing 120 has been described to be rotated in a firstdirection and the blade 108 a has been described to rotate in a seconddirection in response to impact with an object in the environment, insome examples, a housing of a blade assembly is rotatable in both afirst direction and a second direction. The blade is also rotatable inboth the first direction and the second direction. For example, FIG. 18Ashows a schematic top view of a blade assembly 200. In contrast to theblade assembly 106 described herein, the blade assembly 200 is rotatablein both a clockwise direction 202 and a counterclockwise direction 204about a drive axis 206 to cut grass. In particular, blades 208 of theblade assembly 200 include edges 210 a, 210 b both usable to cut grass.

Blades 208 of the blade assembly 200 are mounted in a housing 212 of theblade assembly 200 such that the blades 208 are rotatable relative tothe housing 212 in both a clockwise direction 214 and a counterclockwisedirection 216 about a mounting axis 218. The blades 208 arespring-mounted. The blade assembly 200 includes, for example, springs220 connecting the blades 208 to the housing 212. The springs 220 are,for example, extension or compression springs that extend or compress inresponse to movement of the blades 208 within the housing 212. Absentimpact with objects in the environment, the blades 208 are in neutralpositions as depicted in FIG. 18A.

As shown in a schematic side view of the blade assembly 200 in FIG. 18B,the blades 208 are mounted such that they do not rise within the housing186, e.g., within slots 222 of the housing 212, when the blades 208rotate relative to the housing 212. In this regard, the blades 208 maynot have a tilt angle, as described with respect to the blades 208.Furthermore, the slots 222 are configured to accommodate rotation of theblades 208 relative to the housing 212 in both the clockwise direction214 and the counterclockwise direction 216 about the mounting axis 218.

During mowing operations, the blade assembly 200 is rotatable (e.g., bythe actuator 112) in both the clockwise direction 202 and thecounterclockwise direction 204. The edge 210 a of the blade 208 cuts thegrass when the blade assembly 200 is rotated in the clockwise direction202. The edge 210 a of the blade 208 can contact objects in theenvironment during the rotation of the blade assembly 200 in theclockwise direction 202. In response to impact of the edge 210 a with anobject, the blade 208 rotates in the counterclockwise direction 216relative to the housing 212 such that a radius of the blade tip isreduced. As a result, the blade 208 maneuvers laterally around theobject to avoid being stuck in contact with the object, as described ingreater detail with respect to FIGS. 15A to 15D. The rotation of theblade 208 within the housing 212 in the counterclockwise direction 216causes the corresponding spring 220 to compress. When the blade 208 ismoved beyond the object, the compressed spring 220 biases the blade 208back to the neutral position.

The edge 210 b cuts the grass when the blade assembly 200 is rotated inthe counterclockwise direction 204. The edge 210 b can contact objectsin the environment during the rotation of the blade assembly 200 in thecounterclockwise direction 204. In response to impact of the edge 210 bwith an object, the blade 208 rotates in the clockwise direction 214relative to the housing 212 such that a radius of the blade tip isreduced. As a result, the blade 208 maneuvers laterally around theobject to avoid being stuck in contact with the object, as described ingreater detail with respect to FIGS. 15A to 15D. The rotation of theblade 208 relative to the housing 212 in the clockwise direction 214causes the corresponding to spring 220 to stretch. When the blade 208 ismoved beyond the object, the stretched spring 220 biases the blade 208back to the neutral position. Thus, in the example of the blade assembly200 described in FIGS. 18A and 18B, the blades 208 are able to move intoa retracted position (e.g., the retracted position 176) through rotationin the clockwise direction 214 relative to the housing 212 and throughrotation in the counterclockwise direction 216 relative to the housing212.

Elements of different implementations described herein may be combinedto form other embodiments not specifically set forth above. Elements maybe left out of the structures described herein without adverselyaffecting their operation. Furthermore, various separate elements may becombined into one or more individual elements to perform the functionsdescribed herein.

What is claimed is:
 1. A grass cutting mobile robot comprising: a body;and a blade assembly connected to the body and rotatable about a driveaxis, the blade assembly comprising: two or more blades, wherein, foreach of the two or more blades, the blade is rotatably mounted on amounting axis and includes a cutting portion extending inwardly towardthe drive axis from a blade tip; and a housing to hold the two or moreblades, wherein, for each of the two or more blades, at least a portionof the blade extends upward from the mounting axis, through the housing,and relative to a horizontal ground surface so that, in response to animpact, the portion of the blade is configured to move within thehousing towards the drive axis by rotating about the mounting axis ofthe blade to cause the cutting portion of the blade to move upwardrelative to the horizontal ground surface toward the body and to reducea tip radius defined by the blade tip and the drive axis as the bladetip rotates about the drive axis.
 2. The grass cutting mobile robot ofclaim 1, wherein, for each of the two or more blades, the cuttingportion of the blade comprises a length between 10% and 30% of adistance between the mounting axis and the blade tip.
 3. The grasscutting mobile robot of claim 1, wherein: the housing comprises two ormore slots in which to mount the two or more blades, and, for each ofthe two more slots, a slot extends, from proximate the mounting axis,away from the mounting axis and upward at an incline along a portion ofan outer surface of the housing relative to the horizontal groundsurface.
 4. The grass cutting mobile robot of claim 1, wherein, for eachof the two or more blades, the mounting axis of the blade and the driveaxis are non-parallel.
 5. A grass cutting mobile robot comprising: abody; and a blade assembly connected to the body and rotatable about adrive axis, the blade assembly comprising: blades; a housing to hold theblades, the housing comprising a slot in which to mount a blade of theblades so that a portion of the blade is movable through the slottowards another blade of the blades in response to an impact, and theslot sloping upwards along an outer surface of the housing towards thebody, thereby enabling the blade to move upwards relative to a groundsurface toward the body in response to the impact; and a spring thatconnects the blade to the housing, the spring for constraining movementof the blade relative to the housing.
 6. The grass cutting mobile robotof claim 5, wherein: the spring is a torsion spring having a first endcoupled to the housing and a second end coupled to the blade, thetorsion spring having a twist axis, and the blade is configured torotate relative to the housing about a mounting axis coincident with thetwist axis and non-parallel to the drive axis.
 7. The grass cuttingmobile robot of claim 5, wherein the spring biases the blade away fromthe other blade.
 8. The grass cutting mobile robot of claim 5, wherein,absent the impact, a tip radius of the blade is positioned to rotatethroughout a first radius relative to the drive axis and, in response tothe impact and movement of the blade, the tip radius is reduced toward asecond radius relative to the drive axis, the second radius being lessthan the first radius.
 9. The grass cutting mobile robot of claim 5,wherein the blade comprises a first edge and a second edge connected bya surface, the blade being tilted upward relative to the ground surfaceat a tilt angle such that the second edge is higher than the first edgerelative to the ground surface.
 10. The grass cutting mobile robot ofclaim 9, wherein the tilt angle is between 5 degrees and 10 degrees. 11.The grass cutting mobile robot of claim 5, wherein the blade comprises afirst portion and a second portion, the first portion extending throughthe slot in the housing, and the second portion extending downward awayfrom the first portion.
 12. The grass cutting mobile robot of claim 11,wherein the blade comprises a third portion extending along a radialaxis of the blade assembly.
 13. The grass cutting mobile robot of claim5, wherein a surface of the blade facing the body comprises anembossment extending longitudinally along the surface.
 14. The grasscutting mobile robot of claim 5, further comprising a bumper mounted tothe body, the bumper having a first height relative to the groundsurface, the blade having a second height relative to the groundsurface, and the first height being less than the second height.
 15. Thegrass cutting mobile robot of claim 5, wherein, when the blade assemblyis configured to rotate relative to the body in a first direction, theblade being configured to rotate relative to the body in a seconddirection opposite the first direction in response to the impact. 16.The grass cutting mobile robot of claim 15, further comprising: anactuator mounted in the body to rotate the blade assembly, and one ormore processors configured to detect an increase in an electricalcurrent delivered to the actuator, the increase being responsive to theimpact, and reduce the electrical current delivered to the actuator inresponse to detecting the increase.
 17. A blade assembly for a grasscutting mobile robot, comprising: blades; and a housing to hold theblades and configured for coupling to an actuator of the grass cuttingmobile robot so that the housing is rotatable about a drive axis,wherein, for each of the blades, at least a portion of a blade extendsupward and outward from a mounting axis at which the blade is mounted tothe housing such that the blade is movable upward and towards anotherblade in response to an impact.
 18. The blade assembly of claim 17,wherein: the housing is configured to receive a shaft that connects thehousing to the actuator, and the blade assembly further comprises aretention device within the housing, wherein a portion of the retentiondevice is configured to engage the shaft to lock the housing to theshaft, the portion of the retention device being movable relative to theshaft to disengage from the shaft to release the housing from the shaft.19. The blade assembly of claim 18, wherein the retention devicecomprises a tab portion operable by a user to disengage the retentiondevice from the shaft, thereby releasing the housing from the shaft. 20.The blade assembly of claim 18, wherein the portion of the retentiondevice comprises an arm portion configured to slide along the housingrelative to the drive axis to disengage the arm portion from the shaft.21. The blade assembly of claim 18, wherein the housing comprises asplined cavity configured to mate with a corresponding splined portionof the shaft of the actuator.
 22. The blade assembly of claim 18,wherein the housing is configured to receive the shaft to couple thehousing to a body of the grass cutting mobile robot, the shaft defininga groove therein, and the blade assembly further comprising a retentiondevice configured to engage the groove to lock the housing to the shaft.23. The blade assembly of claim 17, wherein: the housing comprises slotsin which to mount the blades, and, for each of the slots, a slot slopesupward along an outer surface of the housing toward a body of the grasscutting mobile robot, thereby enabling the blade to move upwards inresponse to the impact.
 24. The blade assembly of claim 17, furthercomprising a retention device comprising a plurality of arm portionsconfigured to engage a groove in the shaft to lock the housing to theshaft.
 25. The grass cutting mobile robot of claim 1, wherein the bladeassembly further comprises two or more springs comprising first endscoupled to the housing and second ends coupled to the two or moreblades.
 26. The grass cutting mobile robot of claim 1, wherein, for eachof the blades, the portion of the blade extends upward from the mountingaxis, through the housing, and relative to the horizontal ground surfacewhen the blade is in a fully extended position and when the blade is ina fully retracted position.
 27. The grass cutting mobile robot of claim1, wherein the housing comprises two or more slots through which the twoor more blades move, each of the two or more slots inclined along anouter surface of the housing relative to the horizontal ground surface.28. The grass cutting mobile robot of claim 3, wherein, for each of thetwo or more slots, an angle of the incline relative to the horizontalground surface is between 5 and 10 degrees.
 29. The blade assembly ofclaim 17, further comprising springs that connect the blades to thehousing, the springs for constraining movement of the blades relative tothe housing.